PermaIce. But I think its a little costly. I have done dummy cakes and just used fondant or buttercream. Hold ups good. I have a fondant one i take to wedding shows. It looks good and has been sitting around for about 8 months.

SeriousCakes mentioned something she stumbled upon the other day. She said she's been making practice icing by just whipping up shortening and powdered sugar. No flavoring, maybe a bit of water or milk to thin it. But she thought why put in the extra effort and ingredients if it's just for practice?